A mobile communication system generally includes a base station and a mobile terminal. Each of the base station and the mobile terminal has a transmitting function and a receiving function. In general, a radio link from a transmitter of the base station to a receiver of the mobile terminal is called a “downlink”, and a radio link from a transmitter of the mobile terminal to a receiver of the base station is called an “uplink”.
FIG. 10 is a schematic diagram of a conventional mobile communication system. The conventional mobile communication system includes a base station 13, an antenna for the base station 14, a mobile terminal 15, and an antenna for the mobile terminal 16. The base station 13 transmits a signal to the mobile terminal 15 for a downlink, and receives a signal from the mobile terminal 15 for an uplink, via the base station antenna 14. The mobile terminal 15 transmits a signal to the base station 13 for an uplink, and receives a signal from the base station for a downlink, via the mobile terminal antenna 16.
FIG. 11 is a timing chart of an uplink of a TDMA (time division multiple access) system. FIG. 12 illustrates frequency bands of an uplink and a downlink in the system shown in FIG. 11. The TDMA system is also called a TDM (Time Division Multiplexing)/TDMA system including the uplink and the downlink. The system operations are explained in detail in “TDMA Communications”, written by Heiichi Yamamoto, et al. (Edition of IEICE (The Institute of Electronics, Information and Communication Engineers), Chapter 2, 1989).
FIG. 11 illustrates a state in which six users (i.e., channels) use the same frequency at the same time by dividing a time frame. For example, in frame #A, a communication is conducted based on a time division from #A0 to #A5. In a case of a downlink based on the TDM system, a single base station transmits signals based on the time division. Therefore, there is no influence of a relative timing error attributable to a distance. However, in a case of an uplink based on the TDMA system, since a plurality of different mobile terminals communicates with the base station simultaneously, and distances between the base station and each of mobile terminals are different, a reception timing in a reception channel (i.e., burst) at the base station becomes different. Therefore, since it is necessary to secure an assumed difference between a maximum delay time and a minimum delay time as a guard time in providing services, a time width that each mobile terminal can utilize becomes narrower than ⅙ of a frame length.
Although the problem can be relieved to a certain extent if transmission timing is controlled in each mobile terminal based on a time alignment processing, the processing is quite complicated. Furthermore, the frequency arrangement of an uplink and a downlink based on the TDM/TDMA system is a single signal spectrum as shown in FIG. 12.
FIG. 13 is a block diagram of a communication from a transmitter to a receiver, comprising a transmission signal input terminal 1, a transmitter 2, a transmitter antenna 4, a receiver 5, a receiver antenna 6, and a modulator 100. FIG. 14 illustrates a relation between a transmission signal and a reception signal in a frequency domain in the communication shown in FIG. 13.
The modulator 100 of the transmitter 2 modulates a signal that is input through the transmission signal input terminal 1. The transmitter 2 transmits the modulated signal via the transmitter antenna 4. The signal receives a phase change or an amplitude change due to a fading or the like. The receiver antenna 6 receives the signal, and the receiver 5 demodulates the received signal. When a delay time due to a reflection is ignorable as compared to a symbol period, the reception signal shows the same spectrum as that of the transmission signal, as shown in FIG. 14, although there is a slight variation in the electric field. In other words, when the same data are transmitted in the #A0 burst and the #A1 burst, and even when the receiver combines the two burst signals, it is not possible to effectively suppress a level fluctuation due to the fading.
FIG. 15 is another block diagram of communication from a transmitter to a receiver, comprising a transmitter 2A, a first transmitter antenna 4A, a second transmitter antenna 4B, a first modulator 100A, and a second modulator 100B. FIG. 16 illustrates a relation between a transmission signal and a reception signal in the frequency domain in the communication shown in FIG. 15.
The modulator 100A and the modulator 100B of the transmitter 2A modulate a signal that is input through the transmission signal input terminal 1, respectively. The transmitter 2A transmits the modulated signals through the transmitter antennas 4A and 4B, respectively. The two signals receive a phase change or an amplitude change in different forms due to the fading or the like. The receiver antenna 6 receives the signals, and the receiver 5 demodulates the received signals. In this case, the signals transmitted from the transmitter antennas 4A and 4B receive different fading influences, respectively. The receiver antenna 6 receives the signals in a combined state. As a result, an average electric field of the received signal increases. The signals after being combined have characteristics of a flat fading, in a similar manner to a case that a single antenna transmits the signal. Therefore, it is not possible to make use of a diversity effect.
Therefore, when a delay time due to a reflection is ignorable as compared to a symbol period, the reception signal shows the same spectrum as that of the transmission signal as shown in FIG. 16, in a similar manner to that shown in FIG. 14, although there is a slight variation in the electric field. In other words, when the same data are transmitted in the #A0 burst and the #A1 burst, and even when the receiver combines the two burst signals, it is not possible to effectively suppress the level fluctuation due to the fading.
According to the conventional mobile communication system, unless a complex processing like the time alignment is employed, the frame utilization efficiency is deteriorated due to the guard time. Even when an arrangement is employed that limits the number of users who can utilize the mobile communication system and transmits the same data through a plurality of channels in parallel, it is not possible to make use of the diversity effect. In other words, with the conventional mobile communication system, it is difficult to realize high capacity and high quality communications due to the above reasons.
It is an object of the present invention to provide a radio communication system and a transmitter, which can realize high capacity and high quality communications without employing a complex time alignment processing.